1. Field of the Invention
The present invention relates to a zoom lens system that is suitable for use in an optical instrument such as a digital camera, etc.
2. Description of Related Art
In recent years there has been an increasing need for a zoom lens system in an optical instrument such as a digital camera, etc., to be more compact (miniaturized) and to have a higher optical quality. There is also a very strong demand for miniaturization of the focusing mechanism system and for a rapid focusing operation.
Photographing lens systems which include a wide angle-of-view and are configured of a negative first lens group, a positive second lens group, and a positive third lens group, in that order from the object side, i.e., three lens groups, are known in the art (Japanese Unexamined Patent Publication Nos. H08-304704, 2000-131611, 2004-85600, 2005-181851, 2005-181852 and 2010-204647).
In the photographing lens system disclosed in Japanese Unexamined Patent Publication Nos. 2000-131611 and 2004-85600, a front-focusing method is employed to carryout a focusing operation by moving the entire first lens group in the optical axis direction. However, in such a front focusing method, if the weight of the first lens group, which constitutes a focusing lens group, is large (if the number of lens elements in the first lens group is large), the motor/actuator that constitutes the focusing mechanism system is also enlarged. Accordingly, the diameter of the lens barrel (which includes the zoom lens system of the present invention and the motor/actuator) is enlarged, thereby enlarging the entire zoom lens system.
In Japanese Unexamined Patent Publication Nos. 2005-181851 and 2005-181852, both the second and third lens groups are moved in the optical axis direction to carry out a focusing operation. However, since two lens groups are used as a focusing lens group, the weight of the focusing lens group increases, so that the motor/actuator that constitutes the focusing mechanism system is also enlarged.
Japanese Unexamined Patent Publication Nos. H08-304704 and 2010-204647 employ an internal focusing system in which focusing is carried out by moving the second lens group in the optical axis direction. However, the burden on the motor/actuator that constitutes the focusing mechanism system is still great, and cannot sufficiently cope with a rapid focusing operation.
Furthermore, in each of the zoom lens system disclosed in Japanese Unexamined Patent Publication Nos. H08-304704, 2000-131611, 2004-85600 and 2010-204647, the zoom ratio is less than 2:1, which is insufficient.
Furthermore, in each of the Japanese Unexamined Patent Publication Nos. H08-304704, 2000-131611, 2004-85600, 2005-181851, 2005-181852 and 2010-204647, the air-distance between the surface closest to the image side of the first lens group to the surface closest to the object side of the second lens group is larger than the air-distance between the surface closest to the image side of the second lens group to the surface closest to the object side of the third lens group, at a minimum focal length (short focal length extremity). Accordingly, the axial light rays within the second lens group, which are incident on the second lens group upon diverging from the first lens group, become too high, so that aberration fluctuations, especially fluctuations in spherical aberration and distortion, occur during a focusing operation.